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TRANSACTIONS 


OF  THE 


NEW  YORK    ELECTRICAL    SOCIETY 


MODERN  TELEPHONE  ENGINEERING 


Lecture  delivered  before  the  New  York  Electrical  Society 
February  J4,  1901 


BY 


KEMPSTER   B.  /MILLER 


NUMBER    SIX 


PUBLISHED   BY   THE 

NEW   YORK   ELECTRICAL   SOCIETY 

J20  Liberty  Street,  New  York 


-f  TK6/6? 


MODERN    TELEPHONE    ENGINEERING 


BY 


KEMPSTER,    B.    MILLER. 


Mr.  President  and  gentlemen  of  the  New  York  Electrical 
Society : 

The  subject  which  you  have  assigned  to  me,  "Modern 
Telephone  Engineering,"  is  one  of  such  breadth  that  only 
portions  of  it  can  be  treated  in  a  single  evening,  and  those 
portions  not  with  as  great  an  amount  of  detail  as  I  would 
desire. 

After  careful  consideration,  I  have  concluded  that  the 
subject  matter  which  is  most  likely  to  interest  the  greatest 
number  of  those  present  is  a  discussion  of  a  few  of  the  en- 
gineering problems  arising  in  planning  a  telephone  ex- 
change, and  also  a  description  of  some  of  the  methods  and 
means  employed  in  what  may  be  considered  modern  tele- 
phone practice.  Time  will  not  permit  me  to  deal  with  the 
various  items  in  detail,  and  therefore  these  remarks  should 
be  regarded  in  many  cases  rather  as  suggestions  than  as 
complete  treatments  of  the  various  points. 

The  statements  which  I  will  make  in  the  course  of  the 
evening  will  relate  to  what  I  believe  to  be  good  telephone 
practice,  regardless  of  whether  it  is  the  practice  of  the  Bell 
or  of  the  independent  companies.  The  circuits  shown  are 
those  of  the  Bell  Company,  and  the  apparatus  mostly  that  of 
the  Kellogg  Switchboard  &  Supply  Company,  with  which 
company  I  am  connected.  I  trust  that  I  may  be  pardoned 
for  appearing  partial  to  the  apparatus  of  this  latter  com- 
pany ;  but,  in  the  first  place,  my  brethren  under  the  wing 
of  the  Bell  Company  have  not  exhibited  an  undue  amount 
of  enthusiasm  in  the  matter  of  furnishing  me  with  cuts  and 
data  thereon.  In  the  second  place,  I  have  been  closely 
identified  with  the  design  and  the  manufacture  of  the 
Kellogg  apparatus  and  am  therefore  better  able  to  talk 
of  it.  In  the  third  place,  I  think  that  most  of  the  tele- 
phone engineers  here  are  perhaps  as  familiar,  if  not  more 
so  than  I  am,  with  the  apparatus  of  the  Bell  Company,  and 
therefore  will  be  more  interested  in  that  of  another  make. 
Be  this  as  it  may,  whatever  methods,  circuits  and  apparatus 
are  described  will  be  typical  of  the  telephone  art  as  it  exists 
to-day. 

The  first  problem  arising  when  a  telephone  exchange  is 
to  be  installed  in  a  large  city  is  that  of  the  number,  size  and 
location  of  the  central  office  or  offices.  The  proper  solution 
of  this  problem  depends  in  all  cases  upon  local  conditions 
and  can  be  attained  only  after  much  consideration  and  cal- 
culation. 

If  the  cheapest  were  the  best,  the  telephone  engineer 
would  experience  comparatively  little  difficulty  in  exactly 
determining  the  location  of  the  telephone  office  or  offices,  as 
well  as  the  boundaries  of  the  office  districts  in  any  city,  after 
having  canvassed  the  communities  and  ascertained  the 
topographical  location  of  the  present  and  prospective  sub- 


scribers. The  most  important  consideration  is  that  of  the 
quality  of  service  to  be  rendered.  This  factor,  "quality  of 
service,"  can  not  be  expressed  in  dollars  and  cents ;  it  must 
be  left  to  the  good  judgment  of  the  engineer  to  so  construct 
the  telephone  plant  that  the  demands  made  by  the  public 
are  satisfied  without  making  the  plant  so  expensive  as  to  be 
an  unprofitable  investment. 

Other  matters  to  be  considered  in  the  preliminary  lay- 
out of  a  system  are: 

Annual  cost  of  operation,  including  salaries  of  operators 
and  the  cost  of  current  supply ;  fixed  charges  and  rent, 
interest  and  depreciation  on  the  office  equipment,  interest 
and  depreciation  on  the  wire  plant,  interest  and  deprecia- 
tion on  the  subscribers'  apparatus,  and  annual  cost  of  main- 
tenance. These  factors,  unlike  the  first  mentioned,  are 
subject  to  predetermination  with  a  fair  degree  of  exactness. 
A  brief  discussion  of  each  of  these  various  factors  as  to 
their  bearing  on  the  number  and  location  of  offices  will 
probably  be  of  interest. 

I  think  it  is  a  well  established  fact  that  the  best  telephone 
service  can  be  given  where  all  the  lines  of  the  subscribers  in 
a  city  run  to  a  single  exchange  and  are  there  handled  by  a 
single  multiple  switchboard.  By  this  means  all  calls  for 
connections  by  any  subscriber  may  be  handled  by  a  single 
operator.  Where  the  number  of  subscribers,  however,  is 
too  great  to  be  connected  to  a  single  switchboard,  or  where 
the  distribution  is  such  as  to  naturally  group  them  about 
well  defined  and  remote  centres,  a  greater  number  than  one 
exchange  must  be  installed  and  lines  provided  between 
them  for  connecting  the  subscribers  in  the  different  ex- 
changes. Such  lines  are  called  trunking  lines,  in  distinc- 
tion to  subscribers'  lines,  which  connect  the  subscriber  with 
the  central  office. 

In  most  cases  where  trunking  is  necessary,  most  calls  have 
to  be  trunked.  I  believe  the  percentage  of  trunked  calls  to 
total  calls  for  Greater  New  York  is  in  the  neighborhood  of 
eighty.  A  connection  made  over  a  trunk  line  necessarily 
occupies  more  time  and  thereby  renders  the  service  a  little 
slower.  In  fact,  the  aggregate  time  required  for  connection 
over  a  trunk  line  is  practically  twice  as  great  as  where  ac- 
complished by  a  single  operator  in  a  multiple  board.  The 
fact  that  two  operators  necessarily  handle  a  trunk  con- 
nection necessarily  increases  the  liability  to  error  in  mak- 
ing the  connection,  and  therefore  produces  less  satisfactory 
service.  The  trunking  apparatus  and  circuits,  in  order  not 
only  to  transmit  speech,  but  to  convey  the  requisite  signals 
between  the  operators,  are  necessarily  more  complex  than 
subscribers'  line  circuits,  and  thereby  a  greater  liability 
to  trouble  exists  than  where  no  trunks  are  required. 
So  far,  therefore,  as  the  question  of  quality  of  service  is 


'  A  Lecture  delivered  before  the  New  York  Electrical  Society,  February  14,  1901, 


MODERN    TELEPHONE    ENGINEERING. 


concerned,  there  can  be  no  question  but  that  a  single  large 
office  serving  all  of  the  subscribers  in  a  given  city  is  best. 

Again,  if  we  consider  only  operating  expense,  a  single 
office  serving  the  entire  city  is  best.  First,  because  in  a 
single  office  the  total  number  of  operators  required  is  a 
minimum  (it  has:  been  amply  .proven  by  experience  that 
this  is  so);  second;  in  a  truhking ;' system  the  number  of 
higher  paid  person-;  •t'K.iio'jvr.itoi.'s  will  be  greatly  increased, 
as  there  will  be  for  instance,  a  manager,  a  wire  chief,  a  chief 
operator,  etc.,  for  each  of  the  different  offices;  third,  the 
item  of  current  supply  is  naturally  higher  with  several 
offices  than  with  a  single  large  one,  as  the  power  system 
requires  more  current  on  account  of  the  greater  amount  of 
apparatus  and  also  on  account  of  the  machinery  being  split 
up  into  smaller  units  and  working  with  less  efficiency. 

The  item  of  fixed  charges  and  rent  includes  the  salaries 
paid  to  the  general  management,  the  business  and  engineer- 
ing offices,  as  well  as  rent.  The  former  items  should  not 
differ  materially  whether  one  office  or  a  greater  number 
is  used.  The  question  of  rent,  however,  is  a  little  more 
complex  and  must  be  accurately  determined  for  each  par- 
ticular city.  It  will  vary  from  50  cents  to  $2  per  annum  for 
every  square  foot  of  floor  space.  Where  a  single  office  is 
used  this  is  necessarily  located  somewhere  in  the  center  of 
the  city  where  rent  is  highest.  Where  a  number  of  offices 
are  used,  the  offices  will  not  be  in  the  center  of  the  city,  but 
will  of  necessity  be  at  points  densely  populated.  The 
probabilities  are  that  the  rental,  which  will  also  include 
.light,  heat,  elevator  service  and  janitor  service,  etc.,  will  be 
slightly  cheaper  for  a  large  office  located  in  the  heart  of  the 
city  than  for  several  smaller  offices  located  in  the  most 
densely  populated  portion  of  their  respective  districts. 

The  interest  and  depreciation  on  the  office  equipment  is 
in  most  cases  smaller  for  the  single  office  system,  for  while 
it  is  true  that  a  single  large  multiple  board  costs  more  than 
a  number  of  smaller  multiple  boards  having  the  same  ag- 
gregate capacity  as  the  large  one,  yet  the  addition  of  the 
trunking  apparatus  to  the  smaller  boards,  and  the  fact  that 
the  power  plants,  managers,  wire  chief's,  monitor's  and 
trouble  clerk's  desks  and  similar  apparatus  are  duplicated 
at  each  exchange,  will  probably  throw  the  balance  in  favor 
of  the  single  large  exchange.  This  matter,  however,  would 
need  to  be  determined  for  each  set  of  conditions,  as  it  can 
not  be  said  in  all  cases  the  office  equipment  for  a  single 
exchange  would  be  cheaper  than  that  of  a  greater  number 
of  exchanges  furnishing  service  to  the  same  number  of 
subscribers. 

All  the  factors  so  far  considered ,  that  is,  the  quality  of 
service,  the  cost  of  operation,  fixed  charges  and  rent,  and 
depreciation  on  the  office  equipment,  seem  to  lean  in  favor 
of  the  single  office,  some  of  them  decidedly  so,  others  not 
so  strongly.  Against  these  must  be  put,  in  some  cases,  the 
interest  and  depreciation  on  the  wire  plant  and  the  item  of 
annual  cost  of  maintenance. 

By  wire  plant  is  understood  everything  pertaining  to  the 
telephone  plant,  from  the  subscriber's  premises  to  that  point 
in  the  telephone  office,  or  offices,  where  the  lines  enter  the 
terminal  heads.  It  therefore  embraces  the  underground 
conduits  and  cables,  the  pole  lines  carrying  cables  or  bare 
lines,  together  with  all  necessary  apparatus  such  as  man- 
holes, drop  wires,  etc. 

It  is  easy  to  see  that  the  average  length  of  subscribers' 
lines  will  be  greatest  when  a  single  office  is  used,  and  will 
steadily  decrease  as  the  number  of  offices  is  increased.  It 


has  been  pointed  out  by  Mr.  F.  J.  Dommerque  that  in  a 
given  community  the  average  length  of  subscribers'  lines 
varies  inversely  as  the  square  root  of  the  number  of  offices. 
In  other  words,  if  the  average  length  of  subscribers'  lines 
with  one  office  is  L,  then  the  average  length  of  subscribers' 

lines  with  N  offices  is  approximately  L'  = 


I    N 


This 


law  is  not  strictly  true,  but  gives  a  close  approximation. 
Mr.  Dommerque,  in  planning  the  re-equipment  of  the  sys- 
tem of  the  Chicago  Telephone  Company  on  an  ultimate 
basis  of  fifty  thousand  (50,000)  subscribers,  found  that 
the  average  length  of  line  for  the  city  of  Chicago,  if 
equipped  with  one  exchange,  would  be  twenty  thousand 
(20,000)  feet.  From  this  figure  and  from  the  law  of  varia- 
tion just  stated,  the  curve  shown  in  Fig.  I  has  been 
plotted,  giving  the  average  length  of  subscribers'  line  for 
any  greater  number  of  offices  up  to  sixty.  It  was  found 
that  the  actual  measurements  of  subscribers'  lines  for  the 
various  proposed  layouts  for  the  city  of  Chicago  closely 
coincided  with  the  figures  of  this  curve,  so  closely  in  fact 
that  such  a  curve  was  found  to  be  a  practically  reliable 
source  of  information  for  any  number  of  offices.  A  simi- 
lar curve  might  be  plotted  for  trunk  lines  and  order  wires 
connecting  such  offices,  which  curve  would,  of  course, 


t     4     6      8     10    12    14    16    18  20  22  24  26   28  30  32   34  36   38  40  42  44   46  48   50   52  54  56  56  60 
NUMBER  OP  OFFICES   IS  THE   EXCHANGE 

FIG.  i. — SUBSCRIBERS'  LINE  CURVE. 

start  with  a  zero  ordinate  for  one  office  and  rapidly  rise 
with  the  number  of  offices. 

It  is  interesting  to  conceive  these  curves,  when  carried  to 
their  limits ;  that  is,  when  there  were  50,000  central  offices 
for  50,000  subscribers.  The  curve  of  subscribers'  lines 
would  then  have  a  zero  ordinate,  while  the  curve  for  the 
trunk  lines  would  have  a  maximum  ordinate  which  would 
be  equal  in  feet  to  the  distance  from  one  subscriber's  sta- 
tion, through  all  of  the  other  subscribers'  stations.  This 
means  that  there  would  be  no  subscribers'  lines  whatever, 
but  that  from  each  subscriber's  station  there  would  be  a 
trunk  line  corresponding  to  every  one  of  the  49,999  other 
stations,  as  these  trunk  lines  would  extend  through  all  sta- 
tions. We  have  an  actual  example  of  such  a  system  as 
this  in  the  ordinary  house  telephone  system,  where  every 
subscriber  has  his  own  central  office  and  where  every 
subscriber's  line  extends  to  every  other  subscriber. 

If  there  is  but  one  exchange  in  the  city,  the  proper  loca- 
tion for  it  may  be  found  by  marking  upon  the  map  all  of 
the  subscribers  present  and  prospective  and  then  determin- 
ing the  center  of  gravity  of  the  system,  so  to  speak.  This 
may  readily  be  done  by  drawing  a  straight  line  through 
the  map  in  such  a  manner  as  to  have  one-half  of  all  the  sub- 
scribers on  one  side  of  it  and  the  other  half  on  the  other. 


MODERN    TELEPHONE    ENGINEERING. 


In  a  similar  manner  another  line  may  be  drawn  at  right 
angles  to  the  first,  bisecting  the  total  number  of  subscribers, 
and  the  intersection  of  these  two  lines  will  probably  be 
fairly  close  to  the  proper  place  for  the  central  office. 

A  remote  group  of  subscribers,  not  cut  by  these  two  lines, 
tvould  tend  to  draw  the  true  center  of  distribution  from  the 
point  so  located  toward  it.  In  order  to  avoid  errors  from 
this  source,  a  number  of  other  pairs  of  bisecting  lines  should 
be  drawn,  the  lines  of  each  pair  being  at  right  angles. 
Three  such  pairs  of  co-ordinates,  30  degs.  apart,  would 
locate  three  centers,  probably  close  together,  and  the  center 
of  the  triangles  formed  by  these  centers  is  a  very  close  ap- 
proximation to  the  center  of  distribution  of  all  the  sub- 
scribers. 

With  this  location  determined,  it  is  an  easy  matter  to  de- 
termine the  total  average  distance  of  the  subscribers  from 
it,  and  therefore  the  total  wire-mileage  of  the  subscribers' 
lines.  With  the  average  subscribers'  distances  as  the  base, 
a  curve  such  as  just  shown,  may  be  plotted,  which  will 
show  in  a  close  enough  degree  of  approximation  what  the 
average  length  of  subscribers'  line  would  be  for  anv  other 
number  of  central  offices. 

Of  course,  as  the  number  of  offices  increases,  the  wire- 
mileage  of  the  trunk  lines  and  order  wires  increases  and 
figures  concerning  this  must  be  worked  out  in  consideration 
of  the  locations  of  the  various  offices  and  the  amount  of 
traffic  between  them.  This  latter  factor,  the  traffic  between 
offices,  varies  greatly  according  to  the  kind  of  districts  con- 
nected, and  it  is  therefore  almost  impossible  to  give  general 
figures  as  to  the  mileage  of  trunk  lines  and  order  wires.  It 
may  be  said,  however,  that  the  statistics  of  the  large  cities 
of  this  country  and  Europe  show  that  the  trunk  and  order 
circuit  wire-mileage  is  approximately  one-fifth  of  that  of 
the  subscribers'  wire-mileage.  It  is  evident  that  this  figure 
can  be  used  as  a  basis  for  laying  out  new  exchanges, 
for,  by  the  very  nature  of  things,  there  can  be  no  definite 
ratio  between  the  two. 

If  it  is  found  necessary  for  any  reason  to  divide  the  sys- 
tem up  into  office  districts,  a  great  amount  of  care  must  be 
taken  as  to  the  proper  boundaries  for  these  districts.  Of 
course,  rivers,  water  fronts  and  railroads  will  frequently 
aid  in  the  determination  of  these  boundaries.  There  is  fre- 
quently a  tendency  toward  the  use  of  large  business  streets 
and  thoroughfares  as  boundary  lines  between  districts,  but 
investigation  will  show  that  in  many  cases  this  is  not  good 
practice,  as  such  streets  or  thoroughfares  should  be  the 
centers  of  distribution  rather  than  the  boundaries  of  them. 
Diagonal  thoroughfares  between  offices  should  be  used  as 
trunk  line  routes,  rather  than  boundaries  of  districts,  owing 
to  their  directness  and  to  the  fact  that  a  large  number  of 
subscribers  are  usually  located  in  their  vicinities. 

Having  decided  on  the  boundaries  of  the  districts,  their 
respective  centers  may  be  determined,  and  between  these 
the  most  available  routes  for  trunk  lines  determined. 

In  figuring  the  cost  of  the  wire  plant  for  various  layouts 
of  a  city,  it  will  not  do  to  assume  the  cost  to  be  in  direct  pro- 
portion to  the  wire-mileage  required  by  the  systems  using 
one  office,  or  a  greater  number,  for  there  is  a  factor  enter- 
ing the  problem  which  greatly  favors  the  single  office  or,  I 
might  say,  greatly  favors  the  fewer  number  of  offices.  This 
factor  is  due  to  the  fact  that  with  the  single  office  system,  or 
with  the  system  nsing  few  offices,  a  greater  amount,  in 
proportion,  if  large  conduit  or  cable  may  be  used,  and 
therefore  the  cost  per  conduit  foot  and  per  mile  will  be  con- 


siderably smaller  than  where  a  greater  number  of  offices 
are  used. 

In  general,  it  may  be  said  that  the  interest  and  deprecia- 
tion on  the  wire  plant  decreases  as  the  number  of  offices 
increases,  and  this  is  the  main,  and,  in  fact,  practically  the 
only  item  to  offset  the  advantages  of  the  fewer  number 
of  offices  or  the  single  office. 

The  interest  and  depreciation  on  the  subscribers'  ap- 
paratus would  be  the  same,  whether  one  or  many  offices 
were  used. 

A  limitation  to  the  size  of  switchboards  has  heretofore 
necessitated  a  greater  sub-division  of  offices  in  very  large 
cities,  than  any  of  the  considerations  so  far  pointed  out 
would  warrant. 

Until  recently,  it  has  been  the  policy  of  the  Bell  Tele- 
phone Company  to  establish  no  central  offices  having 
switchboard  capacity  for  over  6000  lines.  This  limit  was 
maintained  because  of  the  fact  that  it  has  not  been  practi- 
cable to  build  multiple  switchboards  having  a  larger  capa- 
city than  that.  This  limit  in  the  capacity  of  the  board  has 
been  due  entirely  to  mechanical,  rather  than  electrical  con- 
ditions, and  on  account  of  it,  it  has  been  impracticable  to 
build  such  exchanges  as  those  of  New  York  and  Chicago 
with  a  single  office,  if  other  conditions  had  warranted  it. 

There  has  recently  developed,  however,  a  strong  tend- 
ency on  the  part  of  the  Bell  and  the  Independent  com- 
panies to  build  central  offices  with  a  much  larger  equipment 
than  6000.  The  Cuyahoga  Telephone  Company,  of  Cleve- 
land, is  now  equipped  with  a  multiple  board  having 
a  capacity  of  nine  thousand  six  hundred  (9600)  lines  in- 
stalled, and  with  an  ultimate  capacity  of  about  twenty 
thousand  (20,000).  The  board  at  Paris,  France,  equipped 
by  the  Western  Electric  Company,  has  a  present  capacity 
of  something  like  nine  thousand  three  hundred  (9300) 
lines,  and  the  Kinloch  Telephone  Company,  of  St.  Louis, 
Mo.,  has  a  present  equipment  of  eight  thousand  eight  hun- 
dred (8800)  lines.  I  am  informed  that  a  number  of 
exchanges  operated  by  the  Bell  Company  are  now 
being  planned  or  built,  having  a  capacity  of  nine  thousand 
(9000)  lines  and  upward. 

I  believe  fully  that  the  future  systems  in  large  cities  will 
be  operated  with  fewer  and  fewer  central  offices  and  con- 
sequently with  larger  switchboards.  This  belief  is  rendered 
tenable  by  the  present  tendency  of  operating  companies 
and  also  recent  improvements  in  switchboards  by  which 
the  mechanical  limitation  of  six  thousand  (6000)  lines  is 
removed  and  by  which  as  many  as  twenty-five  thousand 
(25,000)  lines  can  be  handled  in  a  single  straight, multiple 
switchboard  and  even  as  great  a  number  as  one  hundred 
thousand  ( 100,000)  lines  by  the  use  of  systems  now  shown 
to  be  practicable  for  a  smaller  number. 

The  fundamental  object  of  the  multiple  switchboard  is 
to  place  within  reach  of  every  operator  a  line  terminal  or 
spring  jack,  as  it  is  called,  for  every  subscriber's  line  enter- 
ing the  exchange.  As  is  well  known,  the  switchboard  is 
divided  into  sections,  each  section  being  of  such  size  as  to 
allow  an  operator,  without  undue  exertion,  to  reach  over 
its  entire  surface.  On  each  section  are  placed  a  certain 
number  of  line  signals,  by  which  the  subscriber  is  enabled 
to  attract  the  attention  of  the  operator  and  a  corresponding 
number  of  line  jacks,  by  means  of  which  the  operator  may 
make  the  initial  connection  with  that  line  in  response  to 
such  signals.  These  jacks,  which  are  associated  with  the 
line  signals,  are  termed  answering  jacks.  In  addition  to  the 


MODERN    TELEPHONE    ENGINEERING. 


line  signals  and  answering  jacks  are  provided  what  are 
termed  multiple  jacks,  there  being  one  of  these  on  each 
section  for  every  line  in  the  exchange.  To  express  this  in 
a  little  different  way ;  the  various  lines  entering  the  central 
office  are  divided  up  into  suitable  groups,  these  groups 
terminating  in  line  signals  and  answering  jacks  at  the  vari- 
ous sections  of  the  multiple  switchboard.  In  addition  to 
this,  all  the  lines  entering  an  exchange  are  carried  to  every 
section  of  an  exchange,  each  line  terminating  in  every  sec- 
tion in  a  multiple  jack.  An  operator  seated  at  any  section, 
in  response  to  a  display  of  one  of  her  line  signals,  inserts 
one  plug  of  a  pair  into  the  corresponding  jack,,  and,  having 
ascertained  the  number  of  the  subscriber  desired,  inserts 
a  corresponding  plug  of  the  pair  into  the  multiple  jack  of 
the  subscriber  called  for.  As  every  subscriber's  line  has  a 
multiple  jack  at  every  section  of  the  switchboard,  any 
operator  is  able  to  complete  by  herself  any  connection 
called  for  at  her  board. 

The  size  of  the  section  is  limited  by  the  reach  of  the 
operator,  and  it  is  this  fact  that  places  a  mechanical  limit 
to  the  size  of  an  ordinary  multiple  switchboard.  It  has 
been  found  impracticable  in  most  cases  to  place  the  jacks 
closer  than  on  half-inch  centers,  and  under  these  conditions 
no  more  than  six  thousand  (6000)  of  the  multiple  jacks 
could  be  placed,  in  addition  to  the  other  apparatus,  within 
the  space  afforded  by  one  section. 

Two  distinct  methods  have  been  followed  with  the  intent 
of  removing  this  limit.  Each  has  been  productive  of  excel- 
lent results ;  each  has  been  used  separately,  but  up  to  the 
present  time  they  have  not  both  been  incorporated  into  the 
same  installation.  The  first  of  these  methods  is  an  obvious 
one  and  contemplates  the  reduction  in  the  size  of  the  spring 
jacks  with  a  view  to  getting  more  of  them  within  the  reach 
of  the  operator.  The  mechanical  difficulties  in  this  line 
have  been  great,  owing  to  the  fact  that  the  circuits  of  the 
switchboard  were  so  complicated  as  to  require  three,  four, 
and  sometimes  even  five  contacts  to  each  spring  jack.  The 
Western  Electric  Company  in  the  Paris  board  succeeded  in 
reducing  the  dimensions  of  the  jack  to  three-eighths  of  an 
inch  (§-in.)  centers,  both  horizontally  and  vertically,  and 
thus  rendered  practicable  the  installation  of  a  switchboard 
having  a  capacity  for  considerably  over  nine  thousand 
(9000)  lines.  I  believe  that  there  are  other  boards  which 
have  been  installed  with  success  having  this  size  of  jack, 
and  that  others  are  at  present  in  course  of  construction  for 
large  cities  in  this  country.  This  jack  had  three  contacts, 
and  I  believe  is  the  smallest  that  has  ever  been  put  into  ex- 
tensive use. 

Quite  recently  the  company  with  which  I  am  connected 
has  perfected  a  system  of  circuits  for  a  multiple  switch- 
board so  simplified  that  only  two  contact  points  are  required 
for  each  jack ;  that  is,  merely  a  tip  spring  and  sleeve,  these 
two  contacts  always  standing  open  and  never  engaging 
each  other.  By  the  use  of  this  two-wire  system,  it  is  per- 
fectly practicable  to  build  the  jacks  on  £-in.  centers,  hori- 
zontally and  vertically,  and  thus  place  within  the  reach  of 
a  single  operator  no  fewer  than  25,000  lines. 

The  second  method  of  increasing  the  possible  size  of 
multiple  switchboards  was  devised  by  Mr.  M.  G.  Kellogg, 
and  is  now  working  in  two  large  exchanges  in  this  country. 
In  this  system,  which  is  known  as  the  divided  multiple  sys- 
tem, the  switchboard  and  lines  are  divided  each  into  four 
divisions.  The  switchboard  consists  of  four  multiple 
boards,  all  in  the  same  office,  and  to  these  is  connected  one 


of  the  groups  of  lines,  in  the  same  manner  as  if  they  were 
in  separate  exchanges.  With  this  arrangement,  if  no  further 
provisions  were  made,  it  would  be  possible  for  any  sub- 
scriber in  one  division  to  obtain  a  connection  with  any 
other  subscriber  in  his  own  division,  but  not  with  any  one 
in  any  of  the  other  three  divisions.  In  order  to  enable  any 
subscriber  in,  say,  the  A  division  to  obtain  a  connection 
with  a  subscriber  in  the  B,  C  or  D  division,  means  are  pro- 
vided, not  for  trunking  between  the  divisions,  but  for  en- 
abling the  calling  subscriber  to  signal  directly  to  one  of 
the  operators  in  the  division  in  which  the  line  of  the  called- 
for  subscriber  belongs.  This  necessitates  the  use  of  four 
line  signals  and  four  answering  jacks  for  each  line,  cne  of 
each  in  each  division,  and  of  means  whereby  the  subscriber 
may  display  any  one  of  the  signals  to  the  exclusion  of  the 
others.  Instead  of  trunking  between  exchanges,  therefore, 
the  subscriber  sends  his  call  directly  into  the  division  of 
switchboards  to  which  the  desired  subscriber  belongs. 

By  the  use  of  this  divided  multiple  switchboard  it  is  seen 
that  the  ultimate  capacity  of  any  exchange  is  increased  ap- 
proximately fourfold;  not  quite  this,  however,  because 
the  extra  line  signals  and  answering  jacks  take  up  a  portion 
of  the  room  which  would  otherwise  be  available  for  the 
multiple  jacks. 

The  significance  of  what  I  have  said  concerning  the  two 
methods  of  increasing  the  size  of  multiple  switchboard  will 
now  be  apparent.  If  the  four  division  exchange  were  in- 
stalled on  £-in.  centers,  the  capacity  of  such  an  exchange 
would  be  something  over  90,000  lines  operating  without 
giving  the  operator  as  great  a  reach  as  they  have  at  present 
in  some  multiple  switchboards,  and  between  the  subscribers 
of  this  mammoth  exchange"  there  would  be  no  trunking 
whatever. 

You  may  say  that  the  divided  multiple  system  introduces 
complications  in  the  circuits,  which  is  true !  You  may  also 
say  that  operators  will  find  difficulty  in  handling  the  jacks 
on  ^-in.  centers,  which  may  or  may  not  be  true !  But  with 
the  demand  for  telephone  service  growing  with  an  ever 
increasing  rate  as  at  present,  who  will  say  that  the  100,000- 
l:ne  switchboard  will  not  be  required  in  the  near  future, 
and  who  at  present  can  conceive  of  any  other  way  of  hand- 
ling it? 

During  the  past  five  years  a  complete  revolution  has  been 
worked  in  the  circuits  and  apparatus  of  telephone  ex- 
changes, due  to  the  introduction  of  the  systems  known  un- 
der such  names  as  "common  battery,"  "central  battery''  and 
''central  energy."  These  systems,  when  properly  equipped, 
have  proven  vastly  superior  from  both  an  economic  and 
operating  standpoint  to  the  old  magneto  systems,  wherein 
local  batteries  were  used  for  talking  and  magneto  gen- 
erators for  making  the  calls.  The  local  primary  battery  at 
the  subscribers'  station  has  been  superseded  by  a  large 
storage  battery  at  the  central  office,  which  also  takes  the 
place  of  the  old  magneto  generator.  The  mechanical  signal 
or  drop  has  given  place  to  the  lamp  signal.  By  these  inno- 
vations the  apparatus  at  the  subscribers'  station  nas  been 
rendered  simpler  and  more  compact,  that  at  the  central 
office  more  reliable,  and  the  operation  of  the  entire  system 
has  been  rendered  automatic  as  far  as  the  subscriber  is 
concerned,  and  as  much  so  as  desirable  as  far  as  the  op- 
erator is  concerned.  Such  a  system  employs  batteries  at 
the  central  office  for  furnishing  all  current,  for  signaling  by 
the  subscriber  and  for  the  operation  of  the  talking  ap- 
paratus. These  batteries  are  invariably  secondary,  rather 


MODERN    TELEPHONE    ENGINEERING. 


than  primary,  for  large  exchanges,  and  operate  almost  uni- 
versally at  the  voltage  given  by  ten  cells  in  series,  i.  e.,  20 
to  24  volts,  according  to  state  of  charge. 

For  charging  these  batteries  and  for  furnishing  current 
for  ringing  the  subscribers'  bells  complete  power  plants 
are  required,  these  consisting  of  suitable  generating  units, 
driven  cither  electrically  or  mechanically,  together  with 
suitable  controlling  apparatus.  Most  companies  depend 


KIG.    2.  —  ClkCU 


upon  electrical  power,  the  charging  and  ringing  machines 
being  motor  generators  running  directly  from  city  mains. 

It  may  be  imagined  that  the  failure  of  any  source  of 
energy  in  a  large  telephone  system  is  a  matter  of  most  seri- 
ous moment.  The  cutting  off  of  one  branch  of  the  current 
supply  would  cause  consternation  in  the  entire  business 
organization  of  a  community.  In  order  to  prevent  any  such 
breakdown,  all  portions  of  the  power  plant  are  made  in 
duplicate,  and  where  it  is  possible  duplicate  primary  sources 
of  power  are  also  made  available.  Frequently  the  large 
telephone  companies  employ  their  own  steam  generating 
plant,  which  they  use  constantly,  with  the  current  from 
city  mains  as  a  reserve  supply. 

The  determination  of  the  size  of  the  storage  batteries  is 
the  primary  problem  in  the  engineering  work  in  connection 
with  the  telephone  power  plant,  as,  having  once  determined 
it,  the  size  of  the  charging  machines  and  the  details  of  the 
other  apparatus  are  readily  ascertained.  We  know  in  a 
given  city  approximately  the  number  of  subscribers  to  be 
provided  for  in  the  present  installation,  and  we  make  a 
guess  at  what  the  future  number  will  be.  Past  experiences 
have  proven  that  these  guesses  are  almost  always  too  small, 
for  the  growth  of  telephone  service  in  the  United  States 
has  far  exceeded  expectations.  We  also  know  by  statistics 
from  other  cities  approximately  the  number  of  calls  each 
subscriber  will  make  per  day.  This  figure  ranges  from 
about  eight  calls  per  day  in  smaller  and  less  active  cities  to 
something  like  thirty  calls  per  day  in  our  largest  business 
centers.  Experience  has  also  taught  us  that  the  average 
length  of  time  from  the  time  a  subscriber  takes  his  tele- 
phone off  his  hook  in  making  the  call  to  the  time  the  connec- 
tion is  completed  with  the  subscriber  called  for  will  vary 
not  far  from  six  seconds,  if  the  service  is  prompt ;  and  that 
the  average  length  of  conversation  is  somewhat  less  than 
two  minutes.  Knowing  these  figures  and  the  average 
length,  and  therefore  the  average  resistance  of  the  various 
subscribers'  lines,  and  of  the  connecting  apparatus,  the 
number  of  ampere-hours  required  for  the  subscribers'  cir- 
cuits is  readily  determined.  To  this  must  be  added  the 
number  of  ampere-hours  required  for  the  operators'  trans- 
mitter circuits,  the  supervisory  lamp  circuits,  and  any  other 
circuits  that  may  exist.  The  whole  may  be  reduced  to  a 


certain  number  of  hundredths  of  ampere-hours  per  con- 
nection, a  most  convenient  unit. 

Knowing  the  total  number  of  ampere-hours  required  of 
the  storage  battery  in  a  day,  a  size  of  battery  should  be  se- 
lected having  that  capacity  in  ampere-hours,  and  also  one 
having  a  discharge  rate  in  amperes  equal  to  one-sixth  of  the 
total  ampere-hours  required.  Experience  has  shown  that 
the  maximum  discharge  rate  is  equal  to  about  one-sixth  of 
the  ampere-hours  required  per  day. 

The  circuits  of  a  telephone  exchange  are  of  such  a  com- 
plex nature  as  to  defy  adequate  explanation  in  the  short 
time  available. 

In  the  modern  common  battery  exchange  the  operation 
is  such  that  when  a  subscriber  removes  his  receiver  from 
the  hook,  a  lamp  will  be  automatically  lighted  in  front  of 
the  operator  at  the  central  office,  who  handles  the  calls  of 
the  group  of  subscribers  to  which  this  particular  subscriber 
belongs.  The  operator  in  response  to  this  signal  makes 
connection  with  the  subscriber's  line  by  inserting  a  plug 
into  the  spring  jack  of  that  line,  and  after  having  ascer- 
tained his  wants,  completes  the  connection  with  the  sub- 
scriber called  for  by  means  of  a  second  plug  attached  to 
the  first.  As  soon  as  a  connection  is  thus  made  with  a  sub- 
scriber's line,  the  line  lamp  normally  in  control  of  the  sub- 
scriber is  disassociated  from  the  line,  but  another  lamp  asso- 
ciated with  the  plug  is  placed  under  the  control  of  the  sub- 
scriber, this  lamp  being  known  as  a  supervisory  lamp. 

In  Fig.  2  is  shown  a  complete  line  circuit  in  diagram,  in- 
cluding the  apparatus  at  the  subscriber's  station,  which  is 
shown  at  the  left,  and  the  apparatus  at  the  central  office, 
which  is  shown  at  the  right.  The  apparatus  at  the  sub- 


FIG.  3. — CIRCUIT. 

scriber's  station  consists  of  an  induction  coil,  a  receiver, 
a  granular  carbon  transmitter,  a  two  micro-farad  con- 
denser, a  looo-ohm  bell  and  an  automatic  hook-switch 
mounted  in  a  suitable  manner.  It  will  be  noticed  from 
the  diagram  that  the  circuit  between  the  two  sides  of 
the  line  is  held  open  to  direct  currents  by  the  condenser, 
and  to  all  currents  by  the  hook-switch  contact,  while  the 
receiver  is  on  the  hook.  This  leaves  the  bell  in  operative 
relation  to  the  line,  for  the  purpose  of  receiving  calls ;  the 
path  of  the  ringing  current,  which  current  is,  of  course, 
alternating,  being  through  one  side  of  the  line,  through  the 
bell  and  condenser  to  the  other  side  of  the  line.  When  the 
receiver  is  removed  from  its  hook,  the  circuit  between  the 
two  sides  of  the  line  is  closed,  allowing  the  passage  of 
direct  current  through  the  secondary  of  the  induction  coil 
and  the  transmitter  in  series.  At  the  same  time  another  cir- 
cuit containing  the  transmitter,  the  receiver,  the  primary  of 
the  induction  coil  and  the  condenser  is  closed.  The  ob- 


MODERN    TELEPHONE    ENGINEERING. 


ject  of  this  latter  circuit,  which  is  local  to  the  subscriber's 
station,  will  be  pointed  out  later. 

At  the  central  office,  in  addition  to  the  answering  jack 
and  the  multiple  jacks,  there  are  two  relays,  one  termed 
the  line  relay,  which  is  normally  in  circuit  across  the  line 
in  series  with  the  24-volt  storage  battery  common  to  all 


tip  and  sleeve  strands  of  the  calling  plug  are  connected  by 
the  two  halves  of  the  other  winding  of  the  repeating  coil 
between  which  is  connected  to  the  same  battery. 

This  repeating  coil,  and  there  is  one  of  them  for  each  pair 
of  cords  and  plugs,  has  four  windings,  each  having  ap- 
proximately 3300  turns,  all  of  these  windings  being  on  the 


FIG.    4. — CIRCUIT. 


lines,  and  the  other  termed  the  cut-off  relay,  which  is  in  an 
auxiliary  circuit,  and  is  adapted  to  open  the  circuit  through 
the  line  relay  and  the  battery  as  soon  as  a  plug  is  inserted 
into  any  jack  belonging  to  its  line.  Under  normal  condi- 
tions, both  relays  remain  unoperated  and  the  conditions  are 
as  shown.  As  soon,  however,  as  the  subscriber  removes 
his  receiver  from  its  hook,  the  circuit  between  the  two 
sides  of  the  line  is  made  complete  at  the  subscriber's  station 
and  the  current  at  the  central  office  energizes  the  relay  and 
doses  the  circuit  containing  the  line  lamp  and  the  battery. 
This  lamp  is  located  in  immediate  proximity  to  the  answer- 
ing jack,  and  the  operator,  seeing  it  illuminated,  inserts  a 
plug  into  the  answering  jack  in  response  to  the  call.  This 
plug  is  one  of  many  pairs  within  the  reach  of  the  operator 
and  has  three  contacts,  two  of  which,  the  tip  and  sleeve,  are 
adapted  to  engage  the  two  springs  of  the  answering  jack, 
and  thus  complete  the  talking  circuit.  The  third  is  adapted 
to  engage  the  ring  contact  of  the  jack,  and  being  in  connec- 
tion with  the  live  side  of  the  battery,  causes  a  current  to 
pass  from  this  side  of  the  battery  through  the  ring  contact 
of  the  jack  and  the  coil  of  the  cut-off  relay  back  to  the 
other  side  of  the  battery.  The  energization  of  the  cut-off 
relay  serves  to  open  the  circuit  of  the  line  relay,  as  already 
described,  and  this,  when  operated,  extinguishes  the  lamp. 
One  of  the  cord  circuits  is  now  shown  in  Fig.  3.  In  this 
the  answering  and  calling  plugs  are  shown  at  the  extreme 
left  and  right-hand  portions,  respectively,  each  having,  as 
beforesaid,  three  contacts,  two  of  which,  the  tip  and  sleeve, 
are  connected  by  wires  shown  in  heavy  lines,  which  form 
part  of  the  talking  circuit,  and  the  third  of  which  is  con- 
nected through  the  supervisory  signaling  apparatus  to  the 
live  side  of  the  battery,  as  shown.  The  tip  and  sleeve  sides 
of  the  answering  plug  are  connected  together  through  the 
two  halves  of  one  side  of  a  repeating  coil,  between  which 
is  connected  the  common  battery.  In  the  same  manner  the 


same  magnetic  core.  It  is  evident  that  these  connections 
allow  direct  current  from  the  storage  battery  to  pass  out 
over  the  metallic  circuits  of  the  two  connected  lines  to  ener- 
gize the  transmitters.  A  fluctuation  in  the  current  flowing 
in  either  line  will,  however,  by  induction  between  the  wind- 
ings of  the  repeating  coil,  cause  a  corresponding  fluctuation 
ir,  the  current  of  the  other  line.  In  this  way  conversation 
between  two  connected  lines  is  made  possible,  it  being  by 
induction  rather  than  conduction. 


t  INC     *    CORD 


FIG.    5. — CIRCUIT. 

In  the  sleeve  strand  of  the  cord  of  each  plug  is  connected 
a  supervisory  relay,  which  is  energized,  as  is  readily  seen, 
only  while  current  is  flowing  over  the  line  to  which  the 
corresponding  plug  is  connected,  i.  c.,  while  the  receiver 
of  that  line  is  off  its  hook.  These  relays  control  the  circuit 
of  supervisory  lamps,  by  the  illumination  or  darkness  of 
which  the  operator  is  enabled  to  ascertain  the  condition  of 
the  connected  lines.  On  the  right  of  this  figure  is  shown 


MODERN    TELEPHONE    ENGINEERING. 


the  ringing  and  listening  keys.  The  former  is  a  device  for 
switching  the  alternating  current  generator  into  the  circuit 
of  the  calling  plug  and  its  connected  line ;  the  latter  con- 
nects the  operator's  head  telephone,  which  is  shown  in  the 
center  of  the  diagram,  across  the  two  sides  of  the  cord,  in 
order  to  enable  the  operator  to  communicate  with  either 
subscriber. 

The  operation  of  this  circuit  and  apparatus  can  best 
be  understood  by  referring  to  Fig.  4,  which  shows  two 
subscribers'  lines  equipped  as  already  described,  connected 
by  a  cord  circuit  at  the  central  office.  Consider  the  line 
at  the  left  to  be  that  of  the  calling  subscriber  and  that  at 
the  right  the  called.  In  the  first  place,  the  removing  of  the 
calling  subscriber's  receiver  from  its  hook  illuminates  the 
line  lamp,  which  remains  lighted  until  the  operator  answers 
by  the  insertion  of  the  answering  plug  into  the  answering 
jack.  The  operator,  before  inserting  the  calling  plug  into 
the  jack  of  the  called  subscriber,  must  ascertain  whether 
or  not  the  line  of  that  subscriber  is  already  busy.  This  is 
automatically  accomplished,  for  upon  the  touching  of  the 


lighted  when  one  or  both  of  the  subscribers  hang  up  their 
receivers,  which  act  will,  by  opening  the  circuit  of  the  line, 
cause  the  corresponding  supervisory  relay  to  let  go  of  its 
armature  and  thus  remove  the  shunt  about  its  lamp.  When 
both  lamps  are  thus  lighted,  the  operator  knows  that  the 
connection  is  no  longer  desired,  and  without  further  in- 
quiry pulls  down  the  plugs. 

It  is  evident  that  any  fluctuation  in  the  resistance  of  the 
current  of  a  transmitter  at  one  subscriber's  station  will 
cause  a  corresponding  fluctuation  of  the  current  flowing  in 
the  line,  and  this  fluctuation  will  pass  through  the  two 
windings  forming  one  side  of  the  repeating  coil,  which  will 
act  inductively  on  the  two  windings  forming  the  other  side 
of  the  repeating  coil  and  produce  a  corresponding  fluctua- 
tion in  the  other  line.  This  latter  fluctuation  will  pass 
through  the  secondary  winding  of  the  induction  coil  and 
the  transmitter  at  the  subscriber's  station,  and  induce  a  cor- 
responding fluctuation  in  the  local  circuit  containing  the  re- 
ceiver and  the  condenser. 

One  of  the  functions  of  this  local  circuit,  which  has 


tip  of  the  calling  plug  to  the  ring  contact  of  the  multiple 
jack,  she  will  get  a  click  in  her  head  telephone  if  the  line 
is  busy,  and  silence  if  it  is  free.  If  the  line  is  free,  she  in- 
serts the  plug  to  its  full  extent,  and  presses  her  ringing 
key,  which  rings  the  subscriber's  bell.  As  soon  as  the  call- 
ing plug  is  inserted  the  supervisory  lamp,  shown  at  the 
right-hand  lower  portion  of  the  figure,  will  be  illuminated, 
because  its  circuit  will  be  completed  from  ground  through 
the  battery,  through  the  9O-ohm  resistance  coil  and  the 
lamp ;  thence  through  the  third  strand  of  the  cord,  the  test 
ring  contact  of  the  jack,  and  to  ground  through  the  cut-off 
relay.  This  current  will  not  only  illuminate  the  lamp,  but 
will  operate  the  cut-off  relay  of  the  line  called  for  as  well. 
As  soon  as  the  subscriber  of  this  line  responds,  he  will 
close  the  circuit  between  the  two  sides  of  his  line  and  thus 
allow  current  to  pass  over  the  line  to  operate  the  trans- 
mitter. This  current  will  pass  through  the  supervisory  re- 
lay, which  will  then  be  operated  to  close  the  low-resistance 
shunt  about  the  lamp  and  thus  extinguish  it.  Both  lamps 
are  now  out,  and  the  operator  pays  no  further  attention  to 
the  connection  until  she  sees  one  or  both  of  the  supervisory 
lamps  again  lighted.  The  corresponding  lamp  will  be 


FIG.    7. 

already  been  referred  to,  is  that  of  putting  the  receiver  in 
inductive  relation  to  the  line  without  subjecting  it  to  the 
passage  of  direct  current  through  it,  which  circumstance 
might  tend,  if  the  receiver  were  connected  up  the  wrong 
way,  to  demagnetize  its  magnets.  This  local  circuit  is, 
however,  designed  to  accomplish  still  another  result,  this 
being  the  formation  of  a  comparatively  short  path,  through 
which  the  fluctuations  set  up  by  the  transmitter  may  more 
readily  pass ;  in  other  words,  it  is  supposed  to  act  as  the 
local  circuit  of  a  sort  of  a  booster  to  the  outgoing  telephone 
currents.  The  fluctuation  in  current  caused  by  the  trans- 
mitter pass  through  the  condenser  and  primary  winding  of 
the  induction  coil,  and  thus  induce  on  the  secondary,  which 
is  in  the  line,  fluctuations  which  tend  to  increase  the  fluctua- 
tions directly  produced  in  the  line  by  the  transmitter. 

So  far,  only  such  means  have  been  described  as  would 
admit  of  the  connecting  of  two  subscribers  whose  lines 
terminate  in  the  same  exchange.  Where  several  offices  are 
used  in  the  same  system,  the  connections  between  sub- 
scribers in  different  offices  are,  as  has  already  been  pointed 
cut,  made  by  trunk  lines  extending  between  the  offices. 
These  trunk  lines  are  snoken  of  as  outgoing  or  incoming, 


s 


MODERN    TELEPHONE    ENGINEERING. 


according  to  which  end  of  them  is  referred  to.  Thus  an 
outgoing  trunk  line  at  one  office  would  be  an  incoming 
trunk  line  at  another.  The  distinction  is  obvious.  The 
outgoing  trunk  lines  usually  terminate  in  jacks,  in  the 
regular  sections  of  the  multiple  switchboard,  while  the 
incoming  trunk  lines  usually  terminate  in  plugs  and  cords 
and  special  apparatus,  arranged  at  special  incoming  trunk 
sections,  which  sections  are  provided  with  multiple  jacks  in 
the  same  manner  as  the  regular  sections. 

In  Fig.  5  I  have  shown  the  circuits  of  a  trunk  line. 
The  outgoing  end  of  it  at  one  office  being  shown  at  the 
left  of  the  dotted  line,  while  the  incoming  end,  with  its 


FIG.    8. — LOAD   CURVE. 

accompanying  apparatus,  is  shown  at  the  right.  This  cir- 
cuit is  somewhat  complicated  and  will  be  better  understood 
in  connection  with  Figs.  6  and  7,  which  shows  a  subscriber 
in  one  exchange  connected  through  the  local  cord  circuit  at 
that  exchange,  and  a  trunk  line  with  a  subscriber  at  another 
exchange.  Assuming  that  the  subscriber  at  the  left  has  sent 
in  a  call,  the  operator  at  one  of  the  regular  sections  of  the 
switchboard  will  answer  it  in  the  ordinary  way  by  inserting 
a  plug  into  the  answering  jack,  and  having  learned  that 
the  connection  desired  is  for  a  subscriber  in  another  ex- 
change, she  presses  her  order  wire  key  and  communicates 
over  the  order  wire  with  the  incoming  trunk  operator  at  one 


FIG.    Q. — LOAD   CURVE. 

of  the  incoming  trunk  sections  of  the  branch  exchange. 
The  incoming  trunk  operator  repeats  the  order  and  gives 
the  number  of  an  unused  trunk  line  to  the  operator  at  the 
first  exchange.  The  incoming  trunk  operator  at  the  second 
exchange  then  tests  the  line  in  the  ordinary  way  with  the 
trunk  line  plug,  and  if  it  is  found  free,  she  completes  the 
connection  by  inserting  the  plug  in  the  multiple  jack  of  that 
line.  As  soon  as  this  operator  completes  the  connection, 
she  sets  the  automatic  ringing  key,  shown  just  to  the  left 
of  her  plug,  which  locks  in  the  ringing  position,  because 
its  controlling  relay  is  not  operated.  The  ringing  generator 


is  provided  with  an  interrupter,  each  revolution  'of  which 
switches  the  ringing  current  on  to  the  line  for  one  second 
and  the  24-volt  battery  on  to  the  line  for  five  seconds.  Thus 
the  subscriber's  bell  rings  for  one  second  of  every  six  until 
lie  removes  his  receiver  from  its  hook.  When  he  does  this, 
the  current  from  the  24-volt  battery  or  from  the  ringing 
generator,  whichever  happens  to  be  in  circuit,  passes 
through  the  primary  magnet  of  the  ringing  key,  and  trips 
the  latter  into  its  normal  position.  By  this  means  the  bell 
of  the  subscriber  is  rung  at  intervals  until  he  responds 
without  any  further  attention  on  the  part  of  the  operator. 
The  insertion  of  the  trunk  plug  into  the  jack  of  the  called 
subscriber's  line  allows  current  to  flow  from  the  battery 
through  the  test  relay  and  disconnect  lamp,  shown  at  the 
extreme  lower  right-hand  portion  of  the  figure,  and  thence 
through  the  cut-off  relay  to  ground.  This  does  not  light 
the  disconnect  lamp,  because  that  lamp  is  already  shunted 
by  the  armature  of  another  relay  contained  directly  in  the 
trunk  line  circuit,  this  latter  relay  having  been  operated  by 
the  insertion  of  the  plug  into  the  trunk  line  jack  at  the 


FIG.    12. — FRONT   OF    PARIS   EXCHANGE. 

other  exchange.  As  soon  as  the  ringing  key  is  tripped,  the 
current  from  the  battery  at  the  incoming  end  of  the  trunk 
line  flows  through  the  metallic  circuit  of  the  subscribers' 
line  to  furnish  current  for  talking.  This  operates  the  sup- 
ervisory relay  in  the  incoming  cord  circuit,  and  puts  a 
ground  on  the  sleeve  side  of  the  cord  circuit  at  the  first 
office,  thereby  operating  the  calling  supervisory  relay  at 
that  office,  and  this  shunts  the  corresponding  supervisory 
lamp  and  extinguishes  it,  thus  showing  to  the  regular  opera  - 
tcr  at  the  first  office  that  the  subscriber  has  responded.  As 
soon  as  the  subscriber  called  for  hangs  up  his  receiver,  the 
supervisory  relay  at  the  incoming  trunk  section  lets  go  of 
its  armature  and  removes  the  ground  from  the  sleeve  side 
of  the  trunk  line.  This  causes  the  calling  supervisor]'  relay 
at  the  first  office  to  lef  go  of  its  armature  and  remove  the 
shunt  from  around  the  corresponding  lamp.  When  both 
lamps  of  the  regular  pair  of  cords  at  the  first  office  are 
lighted,  the  operator  at  that  office  pulls  down  the  connec- 
tion. By  thus  removing  the  calling  plug  from  the  trunk 
jack,  current  ceases  to  flow  in  the  trunk  line  relay  at  the 


MODERN    TELEPHONE    ENGINEERING. 


FIG.    10.— PARIS    EXCHANGE. 


second  office,  which  removes  the  shunt  from  the  disconnect 
lamp  at  that  office  and  allows  it  to  be  lighted  for  the  first 
time.  This  is  a  signal  for  the  incoming  trunk  operator 
to  disconnect. 

It  will  be  seen  from  the  above  circuits  that  when  two 
subscribers  are  connected  over  a  trunk  line,  they  are  under 
the  control  of  the  originating  operator.  The  disconnect 
signals  from  both  subscribers  go  direct  to  this  operator,  and 
it  is  not  until  she  has  pulled  down  this  connection  that  the 
disconnect  signal  is  given  to  the  operator  at  the  other  ex- 


change.    The  latter  operator  has  no  means  of  listening 
in  on  the  circuit. 

An  interesting  feature  about  such  trunking  systems  is 
the  means  by  which  the  incoming  trunk  operator  at  the 
second  office  informs  the  calling  subscriber  and  the  regu- 
lar operator  that  line  called  for  is  busy.  When  she  finds 
on  testing  that  a  line  is  busy,  she  inserts  the  trunk  line  plug 
into  a  "busy  back"  jack,  which  jack  has  connected  in  its 
circuit  the  machine  giving  an  interrupted  current  which 
gives  a  peculiar,  familiar,  and,  to  most  subscribers,  an  ex- 


FIG.  M-riTTSBURO    EXCHANGE. 


IO 


MODERN    TELEPHONE    ENGINEERING. 


tremely  disagreeable  sound.  This  sound  is  heard  by  botli 
operator  and  subscriber  at  the  first  office,  and  the  discon- 
nection is  made. 

*         *         *         * 

There  is  probably  no  method  of  determining  the  relative 
activity  of  any  city  at  different  times  of  the  day  or  on  dif- 
ferent days,  so  well  as  by  watching  the  telephone  exchange 
in  that  city  in  operation.  For  keeping  track  of  their  busi- 
ness and  being  able  to  handle  it  to  the  greatest  advantage, 
telephone  companies  usually  make  at  frequent  intervals 
what  they  term  "  peg  counts."  These  show  the  total  num- 
ber of  calls  received  and  connections  made  during  each  of 
the  twenty-four  hours  of  the  day.  These  results,  if  plotted 
in  the  form  of  a  curve,  in  which  the  abscissae  represent  time 
and  the  ordinates  represent  total  number  of  calls  per  hour, 


the  use  of  telephone  service.  From  the  hour  of  6  A.  M. 
each  of  the  curves  rise  rapidly,  corresponding  to  the 
awakening  of  the  city,  reaching  a  maximum  for  the  day  in 
the  neighborhood  of  ten  o'clock,  A.  M.  They  then  fall  off 


FIG.    13. — FRONT   OF   PITTSBURO   SECTIONS. 

show  graphically  and  in  a  striking  way  the  amount  of  tele- 
phone business  being  transacted  at  any  given  hour.  The 
curves  (Fig.  8) ,  show  what  I  believe  has  never  been  shown 
before — that  is,  peg  counts,  taken  the  same  day,  of  two 
opposition  exchanges  in  the  same  city.  These  exchanges 
have  about  the  same  number  of  subscribers,  not  far  from 
6000.  The  dotted  curve  represents  the  telephone  activity 
in  the  Bell  exchange,  while  the  solid  curve  represents  that 
of  the  Independent  exchange.  The  rates  of  the  Bell  ex- 
change are  considerably  higher  than  those  of  the  Inde- 
pendent, and  from  this  fact  we  are  enabled  to  draw  several 
conclusions  from  the  form  of  the  two  curves. 

It  will  be  noticed  that  from  twelve  at  midnight  until  six 
o'clock  both  curves  are  very  low,  and  gradually  fall  until 
three  o'clock,  after  which  they  slowly  rise  until  six.  This' 
shows  conclusively  that  the  hour  of  3  A.  M.  is  that  of  least 
business  activity,  except,  perhaps,  in  those  lines,  such  as 
burglary  and  other  allied  industries  which  seldom  lequire 


FIG.    14. — FRONT   OF    IMTTSHURG   SINGLE   SECTION. 

to  a  minimum,  corresponding  to  the  lunch  hour,  between 
twelve  and  one,  and  again  rise  to  a  second  maximum  be- 
tween the  hours  of  four  and  five  in  the  afternoon,  after 
which  they  rapidly  fall  during  the  evening  hours.  The 
fact  that  the  Independent  exchange  has  cheaper  telephones 


FIG.    17. — REAR   OF    TITTSBURG   SINGLE   SF.CTION. 

than  the  other  will,  perhaps,  account  for  the  fact  that  its 
subscribers  begin  work  a  little  earlier  in  the  morning,  reach 
their  maximum  activity  at  a  little  earlier  hour,  take  a  little 
shorter  time  for  lunch,  go  home  a  little  later  in  the  evening 


MODERN  TELEPHONE  ENGINEERING. 


FIG.    15. — REAR    OF   ST.    LOUIS   SWITCHBOARD. 


FIG.    l6. — REAR   OF   PITTSBURG   MULTIPLE. 


12 


MODERN    TELEPHONE    ENGINEERING. 


and  work  a  little  more  all  night,  all  of  which  facts  are  in- 
dicated by  these  two  curves. 

In  Fig.  9  I  show  two  curves  representing  telephonic 
activity  of  the  same  city  on  two  different  dates.  It  will 
be  noticed  that  the  dotted  curve,  which  represents  a 
Monday,  shows  a  greater  number  of  calls  than  the  solid 
curve,  which  was  taken  on  a  Wednesday.  It  is  usually  true 
that  Monday  is  about  the  busiest  day  in  the  week  in  tele- 


FIG.    18. — LINE   CONSTRUCTION   OLD   ST.    LOUIS    EXCHANGE. 


FIG.    24. — CABLE    HEADS. 

phonic  exchanges.  It  would  be  interesting  to  trace  the 
significance  of  the  different  humps  or  characteristics  of 
these  curves,  but  it  will  be  noticed  that  whatever  hump  is  on 
one  is  present,  or  at  least  suggested,  on  the  other.  The 
pronounced  hump  on  the  dotted  curve  at  the  extreme  right 


is  undoubtedly  due  to  the  theater  business,  between  seven 
and  eight  in  the  evening.  Probably  the  reason  why  this 
was  more  pronounced  on  one  curve  than  on  the  other  was 
that  there  was  a  special  attraction  or  special  set  of  attrac- 
tions, in  the  entertainment  line,  on  the  particular  day  on 
which  the  curve  was  taken.  Such  curves  as  the^e  have 
been  said  by  someone  to  represent  the  pulse  of  a  city,  and  I 
think  that  the  longer  one  thinks  about  it,  the  more  apt 


Fill.    IQ. — PITTSBURG   POWER    PLANT. 


FIG.    2O. — PITTSEURG   STORAGE   BATTERIES. 

this  term  appears.  Every  city  has  a  heart  in  its  telephone 
exchange,  and  this  heart  is  throbbing  with  as  great  a  regu- 
larity as  that  of  any  living  creature. 

I  will  now,  in  conclusion,  show  a  number  of  pictures  of 
telephone  exchanges  and  apparatus,  which  I  think  will  be 


MODERN    TELEPHONE    ENGINEERING. 


FIG.    21. — P1TTSBURG   REPEATING  COII.   RACK. 


of  general  interest,  and  which,  perhaps,  will  make  some  of 
the  statements  which  I  have  made  more  clear  to  those  not 
well  posted  in  telephony.  My  first  view  in  this  series  (Fig. 
10)  is  that  of  the  interior  of  the  exchange  installed  by  the 
Western  Electric  Company,  at  Paris,  France.  This  board 
is  next  to  the  largest  in  the  world,  it  having  a  capacity  of 
something  like  9300  lines. 

The  next  view  (Fig.   n)  is  that  of  the  multiple  board 
installed  for  the  Pittsburg  &  Allegheny  Telephone  Com- 


and  Allegheny,  all  operating  in  conjunction  with  the  ex- 
change here  shown.  The  total  present  equipment  of  these 
offices  is  about  7000  lines. 

Fig.  1 2  shows  the  front  view  of  one  of  the  sections  of  the 
great  switchboard  at  Paris,  shown  in  Fig.  9.  In  this  board 
mechanical  annunciators  of  the  electrically  self-restoring 
type  were  used,  which  can  be  seen  at  the  extreme  top  of  the 
section.  Those  signals  are  automatically  restored  when  the 
operator  inserts  a  plug  into  the  answering  jack,  but  this  ar- 


FIG.    22. — LINE   CONSTRUCTION,    OLD   ST.    LOUIS   EXCHANGE. 

pany,  of  Pittsburg,  Pa.,  by  the  Kellogg  Switchboard  & 
vSupply  Company.  This  board  is  now  wired  for  4080  lines 
and  has  an  ultimate  capacity  of  6000.  It  forms  the  main 
office  of  the  Pittsburg  &  Allegheny  exchange,  there  being 
six  branch  offices  located  in  the  various  parts  of  Pittsburg 


FIG.    23. — CABLE  VAULT,    ST.    LOUIS   EXCHANGE. 

rangement  has  a  marked  disadvantage  over  the  more 
modern  form  of  boards  wherein  the  line  signals  are  located 
in  the  immediate  vicinity  of  the  answering  jacks.  With  the 
drops,  as  shown  in  this  section,  the  operator  had  first  to 
look  to  the  top  of  the  section,  fix  the  number  of  the  signal 


MODERN    TELEPHONE    ENGINEERING. 


FIG.    26. — NEW    ST.    LOUIS   DISTRIBUTING    BOARD,    NO.    I. 


FIG.    27. — NEW    ST.    LOUIS    DISTRIBUTING    BOARD,    NO.    2. 


MODERN    TELEPHONE    ENGINEERING. 


displayed  in  her  mind,  and  then  look  down  and  pick  out  the 
corresponding  answering  jack  in  which  to  insert  the  plug. 
In  the  later  boards,  where  the  line  signal  is  next  to  the  cor- 
responding answering  jack,  she  is  saved  this  change  of 
vision,  and  also  this  mental  operation.  She  simply  plugs 
into  the  jack  next  to  the  signal  displayed  without  any  re- 
gard to  its  number. 

In  Fig.  13  is  shown  a  front  view  of  several  of  the  sec- 
tions of  the  Pittsburg  exchange,  a  complete  view  of  which 


FIG.    25. — OLD   ST.    LOUIS    DISTRIBUTING   BOARD. 

was  just  seen,  and  in  Fig.  14  a  closer-view  of  one  of  the 
sections.  Here  the  multiple  jacks  are  plainly  seen  in  the 
upper  portion  of  the  board,  the  answering  jacks  and  line 
signals  being  shown  below.  One  of  the  key  shelves  is 
raised,  showing  the  wiring  of  the  ringing  and  listening 
keys,  and  also  the  springs  of  these  keys.  Below  this  one 
of  the  front  panels  is  removed,  showing  some  details  of 
the  wiring  of  the  connecting  rack  of  the  section. 

The  next  view  (Fig.  15)  is  that  of  the  rear  of  the 
St.  Louis  switchboard.  The  cables  leading  to  the  mul- 
tiple jacks  are  shown  in  the  upper  portion  of  this  picture, 
and  below  them  the  cord  racks,  the  supervisory  relays 
and  the  running  box  containing  the  vaiious  line  cables  are 
clearly  shown. 

Fig.  16  is  a  similar  view  of  the  rear  sections  of  the  Pitts- 
burg  exchange,  a  good  idea  of  the  multiple  cables  being 
afforded. 

Fig.  17  shows  with  greater  detail  the  rear  of  one  of  the 
Pittsburg  sections.  The  multiple  cables  are  seen  at  the  top 
and  below  these  are  the  relay  boxes,  one  of  which  is  open, 
disclosing  the  supervisory  relays  at  the  top  and  the  line  re- 
lays of  that  particular  position.  In  the  lower  portion  of 
the  picture  is  shown  the  connecting  rack,  to  which  all  of  the 
wiring  of  this  particular  section  is  led. 

In  Fig.  1 8  is  shown  the  turning  section  of  the  multiple 
cables.  This  is  at  the  end  of  the  switchboard  and  shows 
where  the  multiple  cables  are  turned  downward  into  the 
running  box,  where  they  run  along  back  of  the  sections, 
and  are  joined  to  their  proper  answering  jacks  and  signal- 
ing circuits  on  the  connecting  rack  of  each  section. 

In  Fig.  1 9  is  shown  what  might  be  called  the  heart  and 
lungs  of  a  telephone  exchange.  This  is  the  power  plant  of 


the  Pittsburg  &  Allegheny  exchange.  On  the  right  is  seen 
the  power  switchboard,  which  is  of  marble,  and  is  provided 
with  voltmeters  and  ammeters,  circuit  breakers,  and  all 
switches  necessary  for  handling  the  machines  and  batteries. 
In  the  center  of  the  picture  are  shown  the  two  ringing  ma- 
chines and  on  the  right  the  charging  machines  for  charging 
the  storage  batteries. 

I  will  now  show  (Fig.  20)  the  storage  batteries  of 
this  same  exchange,  these  comprising  ten  cells  each  of 
chloride  accumulator,  each  cell  being  of  800  ampere-hours 
capacity.  The  tanks  are  of  wood,  lined  with  lead,  and  the 
floor  and  walls  are  of  tile  laid  in  cement.  At  the  upper 
left-hand  portion  of  the  picture  may  be  seen  the  heavy 
copper  bus-bars  forming  the  discharge  leads  of  these  bat- 
teries. These  are  of  solid  copper,  3  ins.  x  £  in.  in  thickness. 

The  next  view  (Fig.  21)  shows  the  repeating  coils  of  the 
Pittsburg  &  Allegheny  system,  there  being  one  of  these  coils 
for  each  pair  of  cords  in  the  entire  exchange.  These  coils 
are  iron  clad  and  are  enclosed,  as  shown,  in  a  practically  dust- 
proof  cabinet.  It  is  through  these  coils  that  the  current  from 
the  storage  battery  is  led  to  the  various  cord  circuits,  whence 
it  passes  to  the  lines  of  connected  subscribers  to  furnish 
talking  current. 

No  single  feature  of  telephone  work  has  shown  greater 
advancement  than  that  of  outside  line  construction.  In 
place  of  the  overhead  wire,  so  common  a  few  yeais  ago, 
and  still  too  common,  the  lines  in  systems  are  being  placed 
underground  to  a  greater  and  greater  extent.  My  next  pic- 
ture (Fig.  22)  illustrates  a  piece  of  overhead  construction, 
showing  lines  as  they  were  about  to  enter  the  exchange  of  the 
old  system  at  St.  Louis,  Mo.  In  marked  contrast  to  this,  the 


s 


FIG.    28. — KELLOGG    DISTRIBUTING    BOARD. 

next  (Fig.  23)  is  a  picture  which  shows  how  the  subscribers' 
lines  now  enter  the  exchange  building  in  that  same  St.  Louis 
system.  This  is  a  picture  of  an  underground  cable  vault 
and  shows  with  what  care  and  system  the  cables  coming 
in  from  the  street  are  led  into  the  cable  shaft  leading  to  the 
exchange  room. 

The  next  view  (Fig.  24)  shows  a  common  method  of  term- 
inating the  lines  of  telephone  exchanges,  or  rather  termin- 
ating the  outside  construction  work.  The  line  cables  may  be 
seen  leading  up  from  the  lower  part  of  the  picture  These 
enter  the  cable  heads,  which  are,  in  this  case,  cast-iron 
boxes  hermetically  sealed,  and  the  various  wires  of  the 


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MODERN    TELEPHONE    ENGINEERING. 


M 

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MODERN    TELEPHONE    ENGINEERING. 


cables  are  distributed  to  insulated  contact  points  on  the  in- 
side of  the  boxes.  These  contact  points  extend  to  the 
outside  of  the  boxes  through  insulated  bushings,  and  from 
thence  the  circuit  is  led  through  protecting  devices,  after 
which  they  are  again  arranged  into  cables  which  proceed 
from  the  top  of  the  cable  heads  and  are  then  led  to  the 
switchboard  terminals. 

This  brings  up  the  subject  of  distributing  boards.  A 
little  thought  will  show  that  some  means  must  be  provided 
for  distributing  the  various  line  wires  which  enter  the  ex- 
change to  their  proper  terminals  on  the  switchboard,  and  to 


FIG.    32. — SINGLE   CUT-OFF   RELAY. 

enable  such  changes  to  be  made  in  this  distribution  as  re- 
quired.   If  such  provision  were  not  made  and  the  line  cables 
were  run  directly  to  the  switchboard,  the  wires  in  one  100- 
pair  cable  being  led,  for  instance,  to  the  first  position,  and 
the  wires  in  the  second  loo-pair  cable  to  the  second  posi- 
tion, etc.,  it  would  be  necessary  at  any  time  when  a  change 
in  a  subscriber's  number  was  desired,  to  open  the  cable, 
take  out    the   proper   wire    and 
fasten   it    to   one   of   the    other 
cables  leading  to  the  proper  sec- 
tion of  the  board.    The  changing 
about  of  the  various  line  wires 
from  one  part  of   the   board  to 
another  is  a  very  common  occur- 
rence, and  to  do  it  in  the  manner 
j  ust  suggested  would  be  wholly 
impracticable.     In  order  to  pro- 
vide   means    for    systematically 
making  such   changes,  recourse 
is   had   to   what   are  called  dis- 
tributing boards  or  frames.  These 
assume  a  great  variety  of  forms, 
but  the  principle  upon  which  they 

are  designed  is  as  follows:  In  one  portion  of  the  distributing 
board  are  placed  clips  suitably  arranged,  in  which  the  wires 
of  the  line  cables  may  terminate.  On  another  portion  of 
the  distributing  board  is  arranged  another  group  of  clips 
or  connectors  in  which  the  separate  wires  of  the  switch- 
board cables  may  terminate.  We  thus  have  all  connections 
from  the  lines,  and  also  all  connections  from  the  switch- 
board wired  in  a  permanent  manner  to  the  various  con- 
nectors on  the  respective  portions  of  the  distributing 
board.  The  gap  between  the  terminals  of  any  pair  of  wires 
on  the  line  side  of  the  distributing  board,  and  those  of  the 
corresponding  pair  leading  from  the  switchboard  is  filled 
by  means  of  bridle  or  jumper  wires.  The  distributing 
board  is  so  arranged  that  these  jumper  wires  may  be  run 
from  any  pair  of  connectors  on  the  line  side  to  any  pair 
on  the  switchboard  side,  and  if  the  distributing  board  is  a 
good  one,  these  jumper  wires  are  laid  with  perfect  system 
and  may  be  changed  as  often  as  desired. 


In  Fig.  25  is  shown  a  distributing  board  which  is 
happily  a  relic  of  the  past,  but  which  in  its  day  was  dis- 
tinctly superior  to  others  and  may  be  said  to  have  served 
its  purpose  well.  This  picture  was  taken  from  the  inside 
of  the  distributing  board  which  was  in  the  form  of  a  hollow 
square,  one  corner  of  which  is  shown.  This  accommodated 
about  4000  lines  entering  the  old  Bell  exchange  at  the 
corner  of  Fourth  and  Pine  Streets,  in  St.  Louis.  The  line 
wires  were  practically  all  aerial  and  were  brought  to  a  large 
tower  at  the  top  of  the  exchange  building ;  they  were  then 
led  down  in  suitable  cables,  as  shown  in  this  picture,  which 
cables  were  fanned  out  for  attachment  to  the  clips  on  the 
upper  portion  of  the  distributing  board.  In  the  same  man- 
ner the  switchboard  cables  were  led  up  to  the  lower  set  of 
clips  on  the  distributing  board.  Connection  between  any 
line  wire  and  any  switchboard  wire  was  then  completed  by 
means  of  the  jumper  wires,  as  readily  seen. 

It  is  the  object  of  modern  distributing  boards  to  do 
away  with  the  confusion  in  these  jumper  wires  and  to  lead 
them  as  far  as  possible  in  parallel  directions  in  their  courses 
from  the  line  to  the  switchboard  sides  of  the  frames.  My 
next  view  (Fig.  26)  shows  the  switchboard  side  of  the  dis- 
tributing board  of  the  present  main  exchange  of  St.  Louis, 
which  exchange  takes  the  place  of  the  old  exchange,  the  dis- 
tributing board  of  which  was  just  shown.  The  switch- 
board cables  may  be  seen  leading  up  from  beneath.  They 
are  fanned  out  to  the  lightning  arrester  terminals  shown  on 
the  right-hand  portion  of  the  board,  these  terminals  being 
omitted  from  the  left-hand  portion,  which  latter  portion  is 
reserved  for  future  growth.  These  arresters  contain  a 


FIG.    37.— STRIP   OF  JACKS. 

device  which  will  open  the  line  and  ground  it  if  an  ex- 
traneous current  of  sufficient  intensity  to  be  considered 
dangerous  comes  in  over  the  line.  Such  currents  are 
termed  sneak  currents,  and  would  do  great  damage  but 
for  these  little  protectors.  The  protectors  also  include 
_static  arresters,  by  means  of  which  a  high  potential  current 
is  caused  to  jump  to  ground  rather  than  go  on  through  the 
intricacies  of  the  switchboard.  The  jumper  wires  are  led 
from  the  terminals  on  this  side  of  the  distributing  board 
through  iron  rings,  some  of  which  may  be  seen  at  the  ex- 
treme left  of  the  picture'and  to  the'line  side  of  the  distribut- 
ing board,  a  view  of  which  (Fig.  27)  is  now  shown.  The 
terminals  on  this  .side  are  arranged  in  horizontal  rows,  and 
the  jumper  wires  coming  from  the  line  side,  after  passing 
through  their  respective  rings,  are  led  up  or  down  as  the 
case  may  be,  to  the  particular  horizontal  row  on  which  the 
desired  terminal  exists,  and  are  then  led  along  the  cor- 
responding horizontal  shelf  to  the  proper  terminal.  The 


i8 


MODERN    TELEPHONE    ENGINEERING. 


cables  leading  in  from  the  street  are  permanently  con- 
nected to  these  horizontal  rows  of  terminals  and  may  be 
seen  coming  in  at  the  bottom  portion  of  the  board  through 
the  trap  door  in  the  floor. 

The  next  view  (Fig.  28)  shows  a  Kellogg  distributing 
board,  this  particular  one  being  that  at  the  Pittsburg  ex- 
change already  referred  to.  The  peculiar  feature  of  this 
board  is  that  the  line  and  switchboard  terminals,  instead  of 
being  arranged  on  opposite  sides  of  the  frame  work,  as  on  the 
board  just  described, are  arranged  on  alternate  vertical  strips, 
there  being  a  strip  of  switchboard  terminals  and  then  a  strip 
of  line  terminals,  and  so  on.  The  arresters  in  this  case  are 
put  on  the  cable  heads,  which  may  be  seen  in  the  rear  of  the 
view.  Jumper  wires  are  led  up  or  down  from  the  respect- 
ive line  terminals  through  the  large  iron  rings  seen  at  the 
bottom  and  top  of  the  board,  and  then  horizontally  to  the 


FIG.    36. — STRIP   OF   LAMPS. 

proper  strip  of  line  terminals,  and  then  up  or  down  to  the 
proper  terminal  on  this.  The  cables  from  the  cable  heads 
in  the  rear  are  led  up  to  the  line  strips  from  beneath,  as 
clearly  shown,  while  the  cables  leading  to  the  switchboard 
may  be  seen  in  the  iron  running  frame  at  the  top  of  the 
picture. 

I  have  had  a  few  slides  prepared  representing  in  detail 
parts  of  the  apparatus  used  in  some  of  the  latest  Kellogg 
exchanges.  I  show  here  (Fig.  29)  a  subscriber's  automatic 
switch-hook  with  the  receiver  upon  it.  The  next  picture 
(Fig.  30)  shows  the  same  hook  with  the  receiver  removed. 
It  will  be  seen  that  the  long  lever  serves  to  bend  the  central 
spring  into  engagement  with  the  lower  or  upper  set  of  springs 
according  to  whether  the  telephone  is  in  use  or  not.  This 
accomplishes  the  changes  of  circuit  by  which  the  operator 
is  automatically  called  and  by  which  she  is  later  given  the 
signal  to  disconnect,  which  changes  are  made,  of  course, 
entirely  without  the  volition  or  knowledge  of  the  sub- 
scriber. 

Passing  on  to  the  central  office,  I  will  show  (Fig.  31) 
a  strip  of  line  relays  similar  to  the  ones  used  in  the  Pitts- 
burg  exchange.  These  relays  are  mounted  twenty  per 
strip,  as  shown. 

The  next  cut  (Fig.  32)  shows  a  single  line  cut-off  relay. 
The  magnet  of  this  is  enclosed  in  an  iron  shell  which  forms  a 
portion  of  the  magnetic  circuit.  The  armature  of  this  relay, 
when  attracted,  causes  the  two  long  springs  to  break  contact 
with  the  lower  springs,  and  to  make  contact  with  the  two 
upper  springs.  I  may  say,  in  passing,  that  all  contacts  in 
the  best  relays  of  this  description  are  made  of  platinum.' 
These  relays  are  about  i  ^  ins.  in  diameter,  and  the  shells 
are  of  soft  iron  about  y%  in.  thick. 

In  the  latest  Kellogg  exchanges  the  line  and  cut-off  re- 
lays are  mounted  side  by  side  on  the  same  strip.  This 
view  (Fig.  33)  shows  such  a  strip  containing  twenty  of  each 


relays,  the  line  relays  being  the  smaller  of  the  two.  The 
strip  represents  a  unit  of  twenty  lines,  which  unit  is  car- 
ried throughout  the  entire  scheme  of  wiring  in  the  central 
office.  The  next  illustration  (Fig.  34)  shows  the  opposite 
side  of  this  relay  strip,  showing  the  connecting  wires  from 
the  various  terminals.  These  terminals  pass  through  walls 
in  the  iron  mounting  strip  and  are  readily  accessible  for 
soldering  in  the  wires.  In  mounting  the  line  and  the  cut-off 
relay  in  such  close  proximity,  many  of  the  wires  which 
would  otherwise  have  to  be  formed  up  in  cables  are  reduced 
to  the  short,  stiff  bare  wires  clearly  shown  in  this  picture. 
Coming  now  to  the  line  signals  themselves,  in  Fig.  35,  I 
show  a  strip  of  drops,  these  being  mounted  on  J^-in.  centers 
so  as  to  correspond  to  the  spacing  of  the  jacks.  These  were 
used  before  the  adoption  of  the  line  lamp  signals  by  the  Kel- 
logg Company,  a  strip  of  which  signals  is  shown  in  Fig.  36. 

This  strip  is  formed  from  a  single 
piece  of  hard  rubber  about  10  ins. 
long  and  }4  in.  thick.     The  lamps, 
one  of  which  is  shown  in  the  front 
of  the  view,  are  so  arranged  as  to 
slide  into  their  sockets  between  the 
jack  springs  in  such  manner  as  to 
make  the  proper  circuit  connections. 
In  front  of  each  lamp  is  a  little  lens, 
usually    of    opalescent    glass,    this 
lens  being  carried   in  a  spun  brass 
cap.     The  advantages  of   the  line 
lamp  signal  over  the  old  form  of  me- 
chanical signal   is  obvious,  the  principal  ones  being  the 
absolute  freedom  from  complexity,  cheapness,  and  the  fact 
that  they  may  be  removed  instantly  if  found  defective  and 
replaced  by  good  ones.     A  defective  line  lamp  may  be 
taken  out  and  another  one  substituted  in  less  than  one-half 
minute.    Another  beauty  of  the  line  lamp  is  that  when  it 
is  out  of  order  it  may  be  thrown  away,  which  is  much  easier 


FIG.    38.— CORD   AND   PLUG   IN   SECTION. 

than  repairing  a  mechanical  drop.  These  lamps  usually 
operate  at  20  or  24  volts,  take  about  £  ampere  and  give  ap- 
proximately ^  candle  power.  The  calculated  life  of  these 
lamps,  with  their  present  degree  of  perfection  and  with  the 
average  rate  of  use,  is  about  thirty  years,  but,  of  course, 
this  figure  remains  to  be  verified  by  actual  service,  for 
unfortunately  the  telephone  business  is  not  yet  old  enough 
to  prove  the  statement. 

I  now  show  (Fig.  37)  what  is  by  all  odds  the  most  import- 
ant piece  of  apparatus  in  a  telephone  exchange — the  spring 
jack.  This  is  a  strip  of  the  two-point  jack  already  referred 
to,  and  it  will  be  seen  that  they  are  mounted  twenty  to  a  strip 
on  spaces  corresponding  to  those  of  the  lamp  jack.  The 


MODERN    TELEPHONE    ENGINEERING. 


same  jack  is  used  for  both  answering  and  multiple.  This 
strip  is  built  up  of  hard  rubber,  brass  and  German  silver, 
the  front  being  a  solid  piece  of  hard  rubber,  slightly  over 
10  ins.  long,  and  drilled  for  receiving  the  sleeve  contacts. 
The  rear  of  the  strip  is  a  piece  of  hard  rubber  milled  to  re- 
ceive the  line  springs  and  the  rearwardly  projecting  lugs 
from  the  sleeve  contacts.  This  rear  strip  is  reinforced  by 
a  heavy  brass  strip,  into  which  the  screws  holding  the  line 
springs  are  threaded.  In  this  particular  strip  the  jacks 
are  on  |-in.  centers,  the  strip  being  7/16  in.  high.  The 
spacing  with  this  same  construction  is  readily  reduced  to 
|4  in.  Where  a  smaller  spacing  than  this  is  required,  as  in 
a  board  of  extreme  size,  a  somewhat  different  construction 
would  be  used. 

I  have  spoken  throughout  the  evening  of  cords  and  plugs, 
with  the  details  of  which  some  present  may  not  be  familiar. 


FIG.    40. — CABLE   FANNED   OUT. 

The  next  picture  (Fig.  38)  shows  one  Of  each,  the  plug 
being  cut  away  to  expose  the  method  of  attaching  the  cord, 
the  cord  having  its  various  layers  removed  to  better  show 
its  construction.  This  plug  has  two  contact  points  instead 
of  three,  as  described  in  connection  with  Western  Electric 
or  Bell  system.  It  will  be  noticed  that  the  two  strands  of 
the  cord  are  composed  of  twisted  tinsel,  over  which  z  wrap- 
ping of  floss  silk  and  a  braid  of  cotton  are  placed.  Thus 
insulated,  the  two  conductors  are  bound  together  by  a 
wrapping  of  cotton,  after  which  they  are  enclosed  in  a 
spiral  of  brass  wire  for  protection.  Over  the  spiral  of  wire 
are  placed  two  layers  of  linen  braiding  throughout  the  en- 
tire length  of  the  cord,  and  for  about  10  ins.  back  of  the 
plug  a  third  reinforcing  layer  is  put  on.  These  cords  vary 
in  length,  according  to  the  size  of  the  multiple  board,  this 
length  being  anywhere  from  36  ins.  up  to  nearly  100  ins. 


The  next  view  (Fig.  39)  shows  a  strip  of  ten  supervisory 
relays,  these  relays  being  the  ones  used  in  connection  with 
the  cord  circuit  for  controlling  the  supervisory  lamps.  These 
are  iron  clad,  being  made  in  the  same  shell  as  is  used  for  the 
cut-off  relay.  The  armature,  a  disk  of  iron  about  i£  ins.  in 
diameter,  is  suspended  in  front  of  the  poles  of  the  electro- 
magnet by  a  thin  leaf  spring.  The  working  parts  of  the 
relay,  including  the  armature  and  contact  points,  are  en- 
closed in  a  brass  cap,  which  cap  locks  on  the  shell  with  a 
bayonet  joint. 

The  next  and  last  view  (  Fig.  40)  shows  a  few  of  the  cables 
used  in  switchboard  work,  as  they  are  formed  up  in  the  fac- 
tory. Some  of  these  are  made  from  standard  21  and  41- 
pair  cables  in  long  lengths  with  their  ends  formed 
and  laced  to  fit  the  particular  portion  of  the  switchboard 
for  which  they  are  required.  Some  of  the  other  cables  are 
formed  by  hand,  exactly  as  shown,  directly  from  the 
insulated  wire,  which  is  usually  twisted  in  pairs.  These 
cables,  as  you  will  see,  remind  one  more  of  diagrams  of 
nervous  systems,  sometimes  seen  in  medical  books,  than 
anything  else  I  can  think  of. 

The  design  of  telephone  switchboards  and  apparatus  is  a 
matter  requiring  attention  to  almost  infinite  detail.  Besides 
having  a  knowledge  of  the  requirements  of  telephone  serv- 
ice and  a  thorough  appreciation  of  the  engineering  prob- 
lems involved,  the  designer  must  have  a  peculiar  faculty, 
which  has  been  termed  "telephone  sense."  Ability  to  dis- 
tinguish between  apparatus  and  circuits  that  will  work  suc- 
cessfully in  practice,  from  those  that  will  work  only  in  the 
laboratory,  is  not  acquired  by  all ;  and  the  problems  are  so 
remote  from  those  of  other  branches  of  electrical  engineer- 
ing, that  it  is  seldom  that  precedent  taken  from  other 
branches  can  be  found. 

Those  who  have  the  best  interest  of  the  telephone  busi- 
ness at  heart  are  now  designing  their  apparatus  and  cir- 
cuits on  the  basis  that  telephone  systems  in  small  villages 
must  have  as  good  transmission  as  those  in  the  largest 
cities,  with  the  idea  in  view  that  in  the  future  the  whole 
continent  will  be  one  vast  telephone  exchange,  the  various 
large  cities  being  the  main  offices,  and  the  long-distance 
lines,  the  trunk  lines  between  them,  and  the  small  villages 
the  branch  exchanges. 

I  regret  that  time,  together  with  a  lack  of  definite 
information,  prevents  me  from  treating  of  Dr.  Pupin's 
recent  invention,  but  this  subject  is  one  of  such  apparent 
importance,  that  to  devote  less  than  a  whole  evening  to  it 
would  be  to  do  both  the  inventor  and  the  audience  an 
injustice. 


PUBLISHED    TRANSACTIONS 

OF    THE 

NEW    YORK    ELECTRICAL    SOCIETY 


NO.  I.    ELECTRICAL  PROGRESS  OF  THE  YEAR,  1887 

By  JOSEPH  WETZLER,  December  28,  1887. 

NO.  2.    A  PRACTICAL  METHOD  OF  CALCULATING  AND  DESIGNING 

DYNAMOS  AND  MOTORS 

By  FRANCIS  B.  CROCKER,  March  28,  1888. 

NO.  3.    THE  SOCIAL  SIDE  OF  THE  ELECTRIC  RAILWAY 

By  T.  C.  MARTIN,  March  12,  1890. 

NO.  4.    ELECTRICITY  AT  HIGH  PRESSURES 

By  ELIHU  THOMSON,  March  29,  1899. 

NO.  5.    SYSTEMS  OF  ELECTRIC  TRANSMISSION  AND  DISTRIBUTION 

By  CHARLES  PROTEUS  STEINMETZ,  November  16,  1900. 

NO.  6.    MODERN  TELEPHONE  ENGINEERING 

By  KEMPSTER  B.  MILLER,  February  14,  1901. 

NO.  7.    ALTERNATING  CURRENT  MOTORS 

By  CHAS.  F.  SCOTT,  April  25,   1901.     (/«  press.) 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 

Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 


APR 


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LD  21-100m-9,'47(A5702sl6)476 


